The present invention relates to a new and improved method of monitoring sound-conducting media, wherein sound vibrations are transmitted to a sound-conducting medium and are received by such medium, and further, the invention relates to apparatus for the performance of the aforementioned method and to the use of such method.
Under the expression sound vibrations or equivalent terminology there is to be understood vibrations in every frequency range where there is possible transmission through the monitored medium, especially vibrations in the ultrasonic range.
As the sound-conducting media there can be used random solid objects, such as for instance glass panes or windows or metallic walls. Mounted thereat is at least one vibration or oscillation transmitter which delivers the sound conducted through solids, preferably in the ultrasonic range, to the glass pane or metallic wall. Mounted at another location is at least one vibration or oscillation receiver which picks-up the ultrasonic vibrations transmitted through the pane, wall or surface. Connected with the vibration receiver is an electrical evaluation circuit controlling a signal device. The vibration receiver can be arranged at another optional location for instance also directly neighboring the vibration transmitter. According to a special arrangement, the vibration or oscillation transmitter can also simultaneously serve as the vibration or oscillation receiver.
Apparatuses of this type can serve, for instance, as intrusion protection devices to safeguard against a burglar or other intruder from breaking into store windows, glass showcases, or safety deposit vaults. Here, the glass panes or vault walls serve as the sound-conducting medium. The present invention, however, is in no way limited to this field of protecting surface-like or areal objects, rather also can be beneficially employed in the same manner for protecting or monitoring random sound-conducting media, for instance, objects on display in museums or showcases or display cabinets, fenced in areas, or as ultrasonic room protection, wherein the object itself, the material of the fencing or enclosure or the air located in a room serves as the sound-conducting medium and any changes in such room lead to a change in the sound field.
A monitoring arrangement known from German patent publication No. 1,913,161 discloses the transmission of sound in the ultrasonic region conducted through a body to the object to be monitored. The ultrasonic arriving at the measuring location is recorded and upon attenuation of the amplitude an alarm device is activated. An alarm is not only however triggered upon damaging or destruction of the object, rather also when it is touched. In practice, for instance in the case of store windows, where accidental touching of the window is unavoidable, this oftentimes results in triggering of a false alarm.
According to a further heretofore known method as disclosed in German patent publication No. 2,056,015 a protected object, especially a glass plate or pane, is placed into resonance vibration or oscillation by means of a vibration transmitter and the vibrations are tapped-off by means of a vibration receiver. Also in this case, when there is a change in amplitude an alarm signal is triggered. Once again, what is of disadvantage with this construction is that an amplitude change and therefore triggering of an alarm not only occurs when the pane is damaged, but even when it is merely contacted. What is additionally a drawback is that the resonance locations of a glass pane also tend to shift in the presence of temperature changes or when its surface is wetted, as when it rains. Also in this case the vibration amplitude varies, without there being present any real reasons for triggering an actual alarm.
In Swiss Pat. No. 557,068 there is described a method wherein for the most part such drawback can be avoided. In this respect use is made of the recognition that the monitored medium possesses closely situated resonance points in a certain frequency range. For evaluation purposes there is employed the so-called group transit phenomenon which prevails due to the fact that at the region of a resonance point the time-difference of a modulation signal superimposed upon the sound oscillations or vibrations, between the receiver and the transmitter varies extremely markedly even with slight changes in the resonance frequency. Consequently, even slight damage to the medium, resulting in a pronounced displacement of the resonance spectrum can be detected, yet during contact there does not occur any false alarm.
What is disadvantageous with this arrangement is that the employed carrier frequency must be adjusted relatively exactly so that it appears in a frequency range where the resonance points or locations are close to one another. Yet, as a practical matter this is oftentimes extremely difficult to accomplish, particularly if the installation of the equipment is not carried out by specially trained personnel.